1. Field of the Invention
The present invention relates to a holder for a drive piston of a setting tool.
2. Description of the Prior Art
European Publication EP-O 346275 B1 discloses an explosive powder charge-operated setting tool including a piston guide and a drive piston displaceable in the piston guide. The piston guide has radial openings facing the drive piston, and spring-biased braking balls engaging the drive piston. The spring, which applies a biasing force to the braking balls is formed as a ring spring for applying a radially acting, with respect to the piston, biasing force to the braking balls. The ring spring is provided on its inner profile with a bearing surface acting on the braking ball. The bearing surface is inclined to the piston at an acute angle that opens in a direction opposite a setting direction. When the drive piston moves in the setting direction, it entrains the braking balls therewith. The braking balls expand the ring spring, which results in the bearing surface transmitting the radial biasing force to the braking balls. The braking balls are pressed radially against the piston body by the spring washer. Even with a small displacement of the drive piston in a direction opposite the setting direction, the braking effect can be substantially reduced or eliminated, as the braking balls displace in the same direction as the drive piston, unloading the spring washer. After being unloaded, the spring washer does not press any more the braking balls against the piston body. Further, a possibility still remains that the drive piston would be displaced, before ignition or firing of the setting tool, in the setting direction as a result of, e.g., the setting tool being pressed hard against a constructional component. The displacement in the return direction is effected due to cooperation of the ring spring with the braking balls.
U.S. Pat. No. 4,162,033 discloses a setting tool with a braking element that continuously applies a braking force to the drive piston.
An object of the present invention is to provide a piston holder having a simplified design and which would reliably retain the drive piston in its ignition-ready position in the absence of ignition.
This and other objects of the present invention, which will become apparent herein after, are achieved by providing a holder having a return element having a first end that applies pressure to the drive piston, and a second end spaced from the first end, and axial and radial stop means fixedly securable in the setting tool and against which the second end of the return element is displaceable and is elastically deformed upon displacement of the drive piston in the setting direction.
In its unloaded condition, the return element only slightly engages the piston body. The friction force, which is defined by the bearing force, provides for entrainment of the return element in the setting direction upon a slow displacement of the drive piston in that direction, with the second end of the return element abutting a stop arranged ahead of the return element in the axial direction.
The return element, upon being displaced, becomes squeezed, with its end remote from the drive piston trying to expand radially. However, the radial expansion is prevented by the radial stop, and the return element becomes stressed. The bearing force, which acts on the piston body, sharply increases.
When the force, which causes the displacement of the drive piston in the setting direction, disappears, the return element expands axially, and the drive piston can be displaced or is displaced in its initial, ignition-ready position. Thus, the drive piston can be reliably retained in its ignition-ready position even when the setting tool, inadvertently, is pressed too hard against a structural component. When the force, which causes the displacement of the drive position in the setting direction, exceeds a predetermined threshold, upon ignition of firing of the setting tool, the return element becomes almost completely squeezed, and the drive piston slides through. The return element imparts practically no braking force to the drive piston when the drive piston is displaced back into its ignition-ready position, because the return element is entrained by the drive piston and becomes immediately unloaded.
According to a particularly advantageous embodiment of the present invention, the return element is formed with a funnel shape. The funnel-shaped return element opens in the setting direction of the setting tool, with its smaller inner diameter lying at the tip of the funnel, engaging frictionally the drive piston. The base-side edge region of the funnel is received in an annual axial and radial stop. Preferably, the axial stop can be formed as a plastic disc. The plastic disc damps, upon ignition, at least impacts acting on the return element in the axial direction
The funnel-shaped return element can be formed as an annular ring or washer expandable in the radial direction, with its inner circumference being offset axially relative to its outer circumference. The annular washer or disc can be formed of a bendable wire or of spring steel sheet, with the inner circumference being subsequently displaced axially relative to the outer circumference.
For reducing the wear and increase of the service life the following measures can be undertaken. The return element can be provided with a hard material coating formed of TiN, TiC, or diamond-like carbon material. The coating can be applied under relatively cool conditions by, e.g., vacuum metallization, so that the desired characteristics of the return element are not altered by heat treatment.
For forming the annular disc or washer, wire or rope, which is formed of single strands, can be used. This insures use of more wear-resistant materials, without making the return element more rigid. Forming the return element of stranded materials insures that it does not break when only one strand is sheared or breaks. Also, a spring wire having a rectangular cross-section can be used for forming the disc or washer. It becomes also possible to decarbonize the drive piston. This not only increases the service life of the drive piston but permits to make the drive piston weaker than the return element. This increases the service life of the return element, without increasing the wear of the drive piston.
When the return element is formed of a bendable wire, the wire ends can be connected, e.g., with a chain plate. When a flat wire is used, the wire end should be flat-squeezed. With braided wire, squeezing is not necessary. In order to circumvent the connection problem when forming the return element of a bendable wire, the wire can be bent continuously into a shape corresponding to that of the return element forming annular disc. When the wire of a predetermined length is used and helice-like bent, the end connection can be eliminated. In this way, a quasi multi-layered annular disc is formed.
As it has already been discussed above, the return element can be formed of spring steel sheets. Here also, for reducing wear, coating can be provided, as discussed above. The return element-forming disc or washer can be stamped out of a spring steel sheet or be formed by using erosive water jet cutting. In case of a laser beam cutting, alloys with or without checked time response can be used such as, e.g., nickel-chrome-based alloys. Such as nimonic alloys or Inconel(copyright) alloys. Heating does not damage such alloys.
The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to its construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiments, when read with reference to the accompanying drawings.